In order to optimize the design of an aircraft's landing gear to minimize the weight and cost, stress engineers perform detailed design analyses based on the specification for anticipated landing conditions.
The allowable envelope of landing conditions is a multi-dimensional set of conditions related to the speed of descent, attitude, weight, instantaneous accelerations and position of the aircraft during landing. This translates into a multi-dimensional envelope of stresses on different components of the landing gear. If any given landing exceeds the allowable stresses on any given component then that landing gear is deemed to have experienced a “hard landing” and must immediately be taken out of service for additional inspection and potential replacement.
Even though the actual damage to the landing gear is due to a complex combination of landing conditions, the regulatory authorities have attempted to simplify all these conditions into a single factor, the vertical descent rate.
Aircraft, especially transport category aircraft, such as those certified under US federal air regulation part 25, are designed to land at vertical descent rates up to 10 feet per second. The 10 ft/s design case(s) for the aircraft and its landing gear represent a limit load condition. Although certification of the aircraft and landing gear typically requires analysis and testing to descent velocities of 12 ft/s, it is expected that material damage can occur in any landing with a descent rate beyond 10 ft/s. The amount of energy absorbed by the landing gear (damper and structure) and aircraft structure is based on the kinetic energy in the aircraft. This is given by the formula Ek=½mV2. As can be seen, the amount of energy depends on both the mass of the aircraft and the aircraft descent velocity, with the energy varying with the square of velocity, hence its importance to determining landing behaviour.
FAA data, shown in FIGS. 1 and 2, has shown that the variation of landing descent rate is similar to that specified in MIL-A-8866 (the sink rate frequency curve commonly used for landing gear design purposes). As can be seen in the data, landings beyond 10 ft/s occur periodically.
The current process for deciding that an aircraft has had a “hard landing”, and thus may have compromised the safety and integrity of the landing gear, is based on a subjective assessment by the pilot and flight crew. The pilot determines whether a hard landing has occurred based on the pilot's experience and perception of the landing events. Anecdotal evidence suggests that most pilots are conservative in their determination of hard landings and they tend to report landings as ‘hard’ that are actually less than 10 ft/sec. Given the low percentage of landings beyond the design threshold indicated by the FAA data, it is not unreasonable to expect that most pilots have not landed at 10 ft/s, hence they have no baseline experience against which a comparison may be made.
Because of the lack of reliable quantitative data, errors may be made in this assessment. As a result, an aircraft may be grounded unnecessarily, at a considerable cost of time and money, or conversely, a damaged aircraft can continue in service, thus potentially compromising public safety.
Once a pilot has reported a “hard landing” the operator immediately grounds the plane and submits the flight data recorder information, aircraft weight, weather conditions, inspection results and other incident data to the airframe manufacturer and/or the landing gear manufacturer for analysis to determine whether or not that landing did, in fact, exceed the design specification for the allowable operating envelope for landings. Conventional flight data recorders do not take all the information required nor use a high enough data acquisition rate to enable a comprehensive and detailed analysis. As a result, in order to be conservative, the limited data often does not allow the manufacturer to waive the “hard landing” report. Therefore, in many instances, the analysis confirms the “hard landing” and the landing gear must be dispositioned according to SAE ARP-4915 which often requires that the landing gear be replaced and quarantined for six months. In extreme cases, additional airframe inspections may also be required.
The ability to quantitatively assess whether a hard landing has taken place is important to the transportation industry in order to reduce the cost of reported hard landings. However, there is an additional reason to have a hard landing indicator.
In an otherwise perfectly smooth landing, the vertical descent velocity results in a given energy depending on the mass (or weight) of the plane. Thus, the regulatory limit of 10 ft/sec determines the allowable passenger and baggage weight on all flights. If this limit were to be reduced slightly (for example from 10 to 9.5 ft/sec), then the allowable passenger and baggage weight can be increased and the airlines can fill more seats and be more profitable.
Since the allowable vertical descent speed directly affects the airline's profitability, this is clearly an issue that warrants attention.
Although various companies have attempted to implement a hard landing detection system in the past, these have largely failed. There are essentially three primary reasons why these systems have failed.
First, a hard landing is not governed exclusively by the vertical descent velocity, it is governed by a complex multi-dimensional array of landing conditions including the aircraft attitude, and position. Thus, even if the vertical descent velocity was accurately measured, the regulatory authorities may hesitate to reduce the vertical descent velocity limit since it is really a proxy measurement of all the multi-dimensional landing conditions that are not being monitored.
Secondly, it is not really the velocity that dictates the landing force, but the acceleration (Force=Mass×Acceleration or F=MA). Thus, measuring the rate of descent cannot really provide accurate force or load calculations, it is really just a substitute or proxy measurement.
Thirdly, some companies have tried to circumvent the need to measure landing conditions and instead measure landing forces directly. One problem with measuring landing forces is that an accurate system would necessitate installation of a large number of sensors in order to capture the complete multi-dimensional array of loads on each component of each landing gear. Furthermore, each landing gear would need this large number of sensors and many of the sensors would need to be placed in areas that would be highly susceptible to damage, thus rendering the data inaccurate and leading to a non-robust and costly system.